1. Field of the Invention
The present invention pertains to apparatus and methods for detecting the presence or absence of material at a predetermined location. More particularly, the present invention relates to systems for determining the amount of material accumulated at a locale. Apparatus and methods of the present invention find particular application in cases of material storage in containers, such as bins, wherein the amount of material present may be determined by detecting the level of the material in the bin. Systems for adding material to, or removing material from, such a container may be operated, or at least enabled, by the present invention based on the determination by the present invention of conditions within the container.
2. Description of the Prior Art
Several prior art systems for measuring material levels in bins include the use of mechanical probes which extend into the bin. Such a probe is made to vibrate, and the physical characteristics of the elastic disturbances, which are affected by the quantity of material present about the probe, are analyzed in terms of the quantity of material present.
Other prior art devices rely upon the variation of one or more electrical properties affected due to the presence of material at different levels within the bin. Thus, capacitive reactance of one or more probes may be affected to signal the amount of material present. The operation of an oscillator circuit, including a resonant probe, may be affected to operate a control system in response to the presence of material near the resonant probe.
U.S. Pat. Nos. 3,807,231 and 3,935,970 disclose a measuring system and apparatus wherein the reactance of an antenna probe varies as a function of the level of material in the container adjacent the antenna. The antenna reactance controls the frequency of a transmitter signal, which is then combined with a constant frequency reference signal. The frequency of the resulting difference signal is used to operate a material level indicator and to provide control information for operating automatic systems for controlling the material level.
An amplitude-modulated detector circuit is disclosed in U.S. Pat. No. 4,169,543. There, a crystal-controlled oscillator generates a constant-amplitude signal which is communicated through a load resistor to an antenna probe circuit. Variations in the amplitude of this loaded signal, due to the presence of material to be detected about the antenna probe, are reflected in variations in the value of a dc voltage level. The variable voltage level is combined with a reference voltage level and the resulting signal is communicated to a switching circuit. The switching circuit provides an output signal depending on a comparison between the combined signal and a second reference level.
A detector circuit is disclosed in U.S. Pat. No. 4,222,267 wherein an antenna probe forms an arm of an impedance bridge on which is impressed a constant-frequency, constant-amplitude signal generated by an oscillator. Presence of material at the antenna probe affects the impedance of the bridge, and accordingly, alters the amplitude of the signal which is then converted to a dc voltage level. A switching circuit includes a comparator which compares the dc voltage -evel with a reference level, and produces an output signal indicative of the presence or absence of material at the antenna probe. The sensitivity of the switching circuit may be adjusted to accommodate variations in the effect different materials to be detected may have on the impedance of the antenna probe. The switching circuit itself is equipped with a light-emitting diode, and functions as a calibration circuit for the detector circuitry.
U.S. Pat. No. 4,226,118 discloses a frequency-modulated continuous level detector including a switch-adjusted sensitivity control capability and an on-board test circuit for both adjusting the sensitivity of, and zeroing, the detector circuitry. A reference oscillator and an antenna oscillator each generate a signal of a predetermined base frequency, with the antenna oscillator output being impressed on an antenna probe extending into the sensing area. The outputs of both oscillators are compared in a differencer circuit which produces an output signal whose frequency is equal to the difference between the frequencies of the two oscillator circuit output signals. The impedance of the antenna probe is altered in proportion to the extent of the antenna immersed in material in the sensing area to alter the frequency of the output signal from the antenna oscillator accordingly. The differencer output signal may be converted to a voltage level proportional to the extent material is present at the antenna probe.
It is desirable and particularly advantageous in certain applications for an operator to be able to monitor a detection system at a location removed from the bin or other container, for example, whose material level is of interest. Providing such a detection system in the form of a sensing unit located at the bin and a control unit that may be positioned at some distance would permit such operation. However, separation between the sensing unit and the control unit may be limited by the length of electrical connectors over which the sensing signal between the components can practically travel. High frequency sensing signals may be subject to noise and other interference, and so must be communicated over electrically shielded lines, for example. The present invention provides a system which utilizes a sensing signal which may be communicated over extended distances without the need for electrically shielded cable, for example. Additionally, in the event of a break in the communication line between the sensing unit and the control unit, the present invention may assume a configuration in which peripheral equipment, to add material to a container or remove material therefrom, for example, would not be enabled.